Electrodes of the type to which the present invention relates are typically made of tungsten which is powdered, pressed and then sintered. Usually, the so-manufactured blank is then subjected to deformation processes which also renders the material more dense, for example by hammering or rolling. The hammering is radially directed against a cylindrical blank. The referenced De Bie et al. U.S. Pat. No. 4,117,367, and U.S. Pat. No. 4,859,239, Passmore, describe such processes.
Radial hammering of a cylindrical blank deforms the material as it is passed through flat hammer jaws The object of this process is to obtain a uniform reduction of the diameter of the blank while, simultaneously, elongating the material. Rolling or drawing also decreases the diameter and increases the length. A typical reduction by hammering is about 20% of the original diameter for each process step.
Starting from a diameter of about 4 mm, it is possible to deform the material by a drawing process if a still smaller diameter is desired.
U.S. Pat. No. 5,422,539, Chodora, assigned to the assignee of the present application, describes an electrode for a high-pressure discharge lamp and a process of its manufacture, in which the tip of the electrode is, as is customary, shaped by a material removal manufacturing step, for example by turning or grinding. According to the Chodora disclosure, axial hammering, that is, in a compression direction, can give better results, since the tip is additionally rendered more dense.
In accordance with the state of the art, the stability of the grain structure could be influenced only when making the electrode blank. The grain limit boundary structures extend parallel to the axis of the wire or electrode blank, not only in the region of the shaft, but also in the region of the tip of the electrode. If the geometry of the tip of the electrode is generated by the known material removal process, such as turning, for example on a lathe, and/or grinding, then grain boundary structures terminate essentially uniformly distribute at the inclined surface of the tip and end thereat, as shown in FIG. 1.
It was not possible up to now to deflect the grain boundary structures, or direct, or center them towards the axis of the blank in the region of the tip, or in the region toward a flat tap or end surface at the electrode tip. The prior art processes resulted in a substantial quantity of scrap which had to be accepted in connection with material removal machining technology. The scrap resulted from the difference between the initial blank and the shape of the final electrode tip.
The technology of radial hammering or forging permitted manufacture of a tip only with very small conical angles, that is, under 10.degree., with ductile metals, such as iron, for example for sewing needles. Tungsten was considered as too brittle for use in this technology, and it was not considered possible to obtain cone angles over 10.degree..